1. Field of the Invention
The present invention relates to a conveying apparatus for conveying a conveyed object, and more particularly to control of conveyance of a recording medium in a recording apparatus for recording on the recording medium.
2. Description of the Related Art
In recent years, an inkjet recording apparatus has made a remarkable progress, and high image quality printing, high speed printing, and printing with lower operation noise have rapidly progressed. In addition, there has been notable growth in a number of users, and accordingly, the inkjet recording apparatus has been used in various ways. Conventionally, in order to achieve high image quality printing, it is preferable that a sheet is fed from an upper cassette, in which the bending of printing paper is reduced to a minimum. On the other hand, in order to stack a large amount of sheets of ordinary paper for printing, it is preferable that the sheet is fed from a U-turn cassette. With regard to feeding from the upper cassette, efforts have been made so that both high-quality paper and ordinary paper can be used, however, with regard to feeding from the U-turn cassette feeding, enough attempt has not been made in order to use the high-quality paper.
Especially, with regard to the feeding from the U-turn cassette, a conveyance resistance (conveyance load) increases due to an effect of stiffness of the printing paper, because the paper is reversed between feeding and printing. In this situation, in order to carry out printing conveyance of the high-quality paper, it is necessary to reduce the conveyance resistance by expanding the size of a conveyance path in which the sheet is U-turned, or otherwise, it is recommendable to use a high-quality paper of weak stiffness. In addition, there are some cases in which degradation of image quality is permitted to some extent.
A conventional U-turn conveyance mechanism includes a conveyance roller (hereinafter referred to as an LF conveyance roller), which is positioned upstream of a recording head in the vicinity thereof, and a U-turn conveyance roller for conveying a recording paper through a U-turn shaped conveyance path. In the conventional U-turn conveyance mechanism, each of the rollers is gear-coupled by a common conveyance motor to mechanically carry out a synchronized conveyance.
As for a driving motor, a DC motor is often used, in order to simultaneously achieve both silent and high speed printing. A rotation amount of the LF conveyance roller and the U-turn conveyance roller is detected by one encoder sensor, and one driving motor, which is a DC motor, is feedback-controlled on the basis of an output signal outputted by the encoder sensor.
Besides, a recording apparatus in which one conveying medium is conveyed by using a plurality of motors and rollers is adopted by a page printer. In such an apparatus, the plurality of rollers are continuously fed while the conveyance medium is being conveyed. Besides, a tension value of the conveying medium detected by a unit for detecting a tension, which is arranged between the rollers, is controlled to be constant.
In an apparatus as disclosed in the U.S. Pat. No. 6,729,712, in order to carry out a recording operation with a high speed in a serial printer, in a case where the DC motor driving a carriage and a DC motor driving a conveyance section are driven in parallel for a prescribed period of time, a drive startup timing of the DC motor driving the carriage is controlled in accordance with the operation of the conveyance section. In addition, because different sources of driving are employed for a feeding operation, and a printing and conveying operation, the recording paper is synchronously fed to the position at which printing starts, by simultaneously operating a feeding roller, and a printing and conveying roller. This is because action of delivering the sheet from the feeding roller to the printing and conveying roller is necessary. However, in the printing and conveying operation, in which good accuracy is most demanded among operations of the inkjet recording apparatus, no attempt has been made to implement a synchronous feeding by feedback control driving using a plurality of DC motors in the printing and conveying in the relatively long conveyance path, such as U-turn conveyance and the like.
However, in a conventional U-turn conveyance and printing, an effect of the conveyance resistance generated when a sheet of strong stiffness is conveyed in the U-turn path cannot be overcome. As a result, degradation of image quality occurs because a desired amount of sheet cannot be conveyed by the LF conveyance roller. In addition, in a case where the U-turn conveyance path having a large diameter is employed, the size of the apparatus body becomes large, and accordingly, a manufacturing cost is increased. Besides, the apparatus of a large size itself cannot be accepted by its user.
Further, the conveyance resistance (conveyance load) generated due to stiffness of the paper sheet varies much depending on the position in the conveyance path at which the trailing edge of the printing sheet exists. Thus, in a constitution in which one driving motor simultaneously drives both the LF conveyance roller and the U-turn conveyance roller, if the conveyance resistance in the U-turn conveyance path is high, the sheet conveyance amount at the U-turn conveyance roller decreases. Thus, the conveyance resistance applied to the LF conveyance roller increases and results in decrease in the conveyance amount at the LF conveyance roller. In addition, both the U-turn conveyance roller and the LF conveyance roller pull the paper sheet between them. On the contrary, if the conveyance resistance in the U-turn conveyance path is low, the sheet conveyance amount at the U-turn conveyance roller increases. Thus, the conveyance resistance applied to the LF conveyance roller decreases and results in increase in the conveyance amount at the LF conveyance roller. In addition, both the U-turn conveyance roller and the LF conveyance roller push the paper sheet between them. Further, there is much difference in the conveyance resistance between when both the U-turn conveyance roller and the LF conveyance roller convey the paper sheet and when only the LF conveyance roller conveys the paper sheet after the trailing edge of the sheet passes the nip of the U-turn conveyance roller. That is, the conveyance resistance changes significantly at the boundary of the regions. Therefore, the conveyance amount by the LF conveyance roller varies correspondingly, which causes remarkable image unevenness.
Further, because the same form of conveyance is employed for both the paper sheet of strong stiffness and the paper sheet of weak stiffness, it is difficult to use both types of paper sheet in a manner suitable to both types.
In addition, in a conveyance form which includes a constant feed control having the tension detection section, an apparatus such as the serial printer performs an intermittent feeding in which stop and startup is repeated. However, in the intermittent feeding, a good and sufficient control cannot be achieved because the cost of the tension detection section is increased, and additionally the period of variance of the tension is short, and its variance is abrupt.
In addition, as a method for synchronously operating a plurality of driving sources which can easily be thought of, there is a method in which a synchronization control of the plurality of motors is carried out by feed-forward control of a pulse motor. In this case, in conveying one sheet of recording paper with two motors, external disturbance is applied to the sheet because each of the motors performs driving regardless of the state of the other motor. Thus, it becomes difficult to obtain an appropriate conveyance accuracy.
Further, in a case of simultaneously operating the DC motors, operation errors are relatively permitted even when there occurs difference in the operation of the motors.
The above problems have been getting more difficult to overcome under the situation in which printing on a thick high-quality printing paper with strong stiffness is demanded while high image quality printing has been recently developed, because of the effect of external force due to bending stiffness and the friction resistance of the printing paper.